Quiescent neural stem cells support continuous, lifelong neurogenesis in specific regions of the adult mammalian brain. The largest adult neurogenic region is the ventricular-subventricular zone (V-SVZ), which lines the entire lateral wall of the lateral ventricles. Quiescent neural stem cells (qNSCs) enter the cell cycle (activate) and give rise to new neurons during homeostasis and regeneration, suggesting they can potentially be harnessed for regenerating the brain after neurodegenerative disease, stroke, and injury. Defining the signals that regulate NSC quiescence and activation is essential to unlock their potential for regenerative medicine. NSCs residing in specific regions of the V-SVZ give rise to distinct subtypes of olfactory bulb interneurons. It is unknown whether quiescence-regulating signals map onto the regional heterogeneity of NSCs, and might thereby underlie the production of distinct interneuron subtypes. A major limitation to our understanding of the regulation of NSC quiescence has been the lack of specific markers to identify qNSCs, and prospectively purify them from their in vivo niche.
Using a novel fluorescence-activated cell sorting (FACS) strategy that allows the purification of qNSCs from the adult mouse V-SVZ niche for the first time, I performed in vitro screens for quiescence-regulating signals. Unexpectedly, neurotransmitters emerged as the main class of qNSC-activating signals, including dopamine, GABA, serotonin, acetylcholine, and opioids. Local and long-range neurons that use these neurotransmitters innervate the V-SVZ in unique regional patterns, suggesting these signals map onto the regional heterogeneity of NSCs. Consistent with this hypothesis, infusions of cholinergic agonist and antagonists into the lateral ventricle resulted in regional changes in NSC proliferation. Moreover, cholinergic antagonists blocked the activation of qNSCs during regeneration, providing evidence that neurotransmitter signaling activates qNSCs in vivo. I then showed that hypothalamic Pomc-expressing neurons innervate the anterior-ventral V-SVZ and promote the activation of Nkx2.1+ qNSCs.
Ablation of Pomc+ neurons resulted in decreased proliferation of NSCs in the anterior-ventral, but not anterior-dorsal, V-SVZ. Moreover, both the activity of Pomc+ neurons, and the proliferation of Nkx2.1+ NSCs in the anterior-ventral V-SVZ decreased in fasted animals, suggesting that hunger and satiety states regulation the generation of a single olfactory bulb interneuron subtype. Indeed, ablation of Pomc+ neurons resulted in a loss of the subtype of olfactory bulb interneuron that is generated by Nkx2.1+ NSCs. Together, my findings suggest that both local and long-range neurons regionally innervate the V-SVZ and mediate neural stem cell activation from the quiescent state.
